New Way to Detect Heartbeat with Magnetic Mini-Sensors

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A miniature magnetic sensor using a tiny cloud of atoms has
successfully tracked a human heartbeat for the first time.

In a new study, researchers from the National Institute of
Standards and Technology (NIST) and the German national metrology
institute tried out the sensors – which until now have been
operated mostly in physics laboratories – in a near-clinical
setting.

The new experiments were carried out at the Physikalisch
Technische Bundesanstalt (PTB) in Berlin, Germany, in a building
described as having the world's best magnetic shielding –
necessary to block the Earth's
magnetic field and other external sources from interfering
with the high-precision measurements.

The NIST sensor – a tiny container of about 100 billion rubidium
atoms in gas form, a low-power infrared laser, and optics –
measured the heart's magnetic signature in picoteslas
(trillionths of a tesla). The tesla is the unit that defines
magnetic field strength.

For comparison, the Earth's magnetic field is a million times
stronger (measured in millionths of a tesla) than a heartbeat,
and an MRI machine uses fields several million times stronger
still (operating at several tesla).

In the experiments at PTB, the NIST sensor was placed 0.20 inches
(five millimeters) above the left chest of a person lying face up
on a bed. The sensor successfully detected the weak but regular
magnetic
pattern of the heartbeat.

The same signals were recorded using the "gold standard" for
magnetic measurements, a SQUID (superconducting quantum
interference device). A comparison of the signals confirmed that
the NIST mini-sensor correctly measured the heartbeat and
identified many typical signal features.

The NIST mini-sensor generates more "noise" (interference) in the
signal but has the advantage of operating at room temperature,
whereas SQUIDs work best at –452 degrees Fahrenheit (-269 degrees
Celsius) and require more complicated and expensive supporting
apparatus.

A spin-off of NIST's miniature
atomic clocks, NIST's magnetic mini-sensors were first
developed in 2004. Recently, they were packaged with fiber optics
for detecting the light signals that register magnetic field
strength.

In addition, the control system has been reduced in size, so the
entire apparatus can be transported easily to other laboratories.

The new results, described in the journal Applied Physics
Letters, suggest that NIST mini-sensors could be used to make
magnetocardiograms, a supplement or alternative to
electrocardiograms.

Further tests of the NIST atom-based magnetic sensors at PTB are
planned and could confirm the potential for more biomedical
applications.